Acoustic resonators are widely incorporated in electronic devices to implement signal processing functions. For example, cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers and other mobile devices may use acoustic resonators to implement frequency filters (e.g., band pass filters) for transmitted and/or received radio frequency (RF) signals. Such filters include ladder filters, for example, having electrically connected series and shunt acoustic resonators formed in a ladder structure. The filters may be included in a duplexer, for example, connected between a single antenna and a receiver and a transmitter for respectively filtering received and transmitted RF signals.
Different types of acoustic resonators can be used according to different applications, including bulk acoustic wave (BAW) resonators, such as thin film bulk acoustic resonators (FBARs) and solidly mounted resonators (SMRs). An FBAR, for example, includes a piezoelectric layer between a first (bottom) electrode and a second (top) electrode formed over a cavity in a substrate. FBARs resonate at GHz frequencies, and are thus relatively compact, having thicknesses on the order of microns, and length and width dimensions of hundreds of microns.
Acoustic resonators used as band pass filters (such as ladder filters, mentioned above) have associated passbands that provide ranges of frequencies permitted to pass through the filters. For example, a multiplexer formed by two filter circuits (which may be referred to as a duplexer) accommodates two signal paths (e.g., a receive path from a common antenna to a receiver and a transmit path from a transmitter to the common antenna). Each of the filter circuits is a band pass filter with a corresponding passband. Accordingly, the receiver is able to receive signals through a receive frequency passband, and the transmitter is able to send transmit signals through a different transmit frequency passband, while filtering out the other frequencies. Other types of filters may have more or fewer than two filter circuits and signal paths, depending on various factors, such as the number signals and their respective frequencies that are to be filtered.
The receive and transmit signals may be RF signals corresponding to various predetermined wireless communication standards, such as universal mobile telecommunications system (UMTS), global system for mobile communication (GSM), wideband code division multiple access (WCDMA), Long-Term Evolution (LTE) and LTE-Advanced, for example. The communication standards identify separate frequency bands for transmitting and receiving signals. For example, LTE is allocated various 3GPP bands, including bands 25 and 41, where band 25 provides a transmit (uplink) frequency band of 1850 MHz-1950 MHz and a receive (downlink) frequency band of 1930 MHz-1995 MHz, and band 41 provides a transmit/receive frequency band of 2496 GHz-2690 GHz. Accordingly, a multiplexer operating in compliance with a 3GPP standard would include filters having passbands within the corresponding transmit and receive frequency bands. Throughout this disclosure, a high frequency edge of a frequency band may be referred to as the upper corner (or upper corner frequency), and a low frequency edge of a frequency band may be referred to as the lower corner (or lower corner frequency) of the frequency band.
Each of the frequency bands may be divided into a number of channels, each channel having a carrier center frequency and a channel bandwidth. Depending on implementation of the frequency band, e.g., based on required data rate, the channels may have different channel bandwidths. The wider the channel bandwidth, the fewer channels are available for use within the frequency band. During operation, mobile devices are allocated a particular channel to help avoid interference with other mobile devices using the same frequency band. For example, in a cellular network, a base station may allocate a different channel to each cellular telephone being serviced by that base station.
Generally, the RF signals sent through channels having corresponding center frequencies close to the upper and lower corners of the frequency band tend to have lower quality than the RF signals sent through channels having corresponding center frequencies closer to the center of the frequency band. For example, in the transmit chain, either the signal-to-noise ratio (SNR) is degraded causing lower data rates or more input power must be provided to the filter to achieve the same output power. In the receive chain, the SNR simply is reduced at the band edges (upper and lower frequency corners) where the insertion loss is larger. As the channels having center frequencies becoming closer to the edges of the passbands of the band pass filters in the cellular phones, these channels have worse insertion loss and variation across the band and also larger group delay and group delay variation, both of which contribute to degraded error vector magnitude (EVM). EVM is a measure of signal distortion that causes more errors in digital signal transmission. Therefore, the band pass filters in cellular phones or other mobile devices allocated channels near the upper and lower corners of the frequency band experience higher insertion loss, resulting in degraded signals and low quality communications, than the insertion loss of the band pass filters in mobile devices allocated channels at or closer to the center of the frequency band.